The present invention relates generally to a cellular communications system including an airborne repeater, and particularly to compensation of airborne repeater links.
The increasing need for communications networks and capabilities in outlying and geographically diverse locations has created greater demand for cellular systems. Many new carriers providing the infrastructure for such systems have focused their resources on building as many terrestrial cell stations as possible to expand their respective areas of coverage and consequently generate more revenue.
However, the buildout rate for the terrestrial cell stations is typically slow and expensive, especially in mountainous or otherwise difficult to access areas. In addition, in some these areas, a carrier""s return on investment may not provide the incentive necessary for the carrier to build the necessary cell stations, thereby leaving these areas with either limited or no cellular service at all. Further, many areas having a sufficient number of cellular communications base transceiving stations to handle calls during both off-peak and peak times cannot adequately handle large volumes of calls during sporting events or other short-term special events that temporarily attract large crowds.
In response to the above, airborne cellular systems have been proposed in which a cellular repeater mounted in an airplane, executing a predetermined flight pattern over a geographic area requiring cellular coverage, links calls from cellular phones within the geographic area to a terrestrial base station. Because the airplane is capable of traversing geographic obstacles and takes the place of the cell stations, such a system overcomes the above-mentioned limitations of conventional terrestrial cellular systems.
Despite its many advantages, an airborne cellular system presents design and implementation problems not present in the design and implementation of conventional terrestrial cellular systems. For example, an airborne cellular system requires both a high frequency feeder link to link the base station and the system switch to the airborne repeater, and a subscriber or user link to link the airborne repeater to cellular phones within the area of coverage. As a consequence of the motion of the airplane relative to the base station or cellular phone, an often significant amount of Doppler shift is introduced on the links. As a cellular system such as a TDMA EIA 136 system is sensitive to Doppler shift characteristics, its performance is degraded generally in proportion to the amount of Doppler shift present.
In addition, as the airborne repeater moves as the plane executes its flight pattern, the communication path link distances between the base station and the airplane and the airplane and the system subscribers constantly change. These changes in path link distances cause signal loss to vary. Also, airplane pitch, roll and yaw can move a beam off of its peak gain, thereby increasing the average power consumption of the repeater and associated equipment, increasing dynamic range requirements and increasing the dynamic range and power consumption of the repeater and associated equipment. Consequently, heavier, more expensive and higher power consumption power amplifiers must be used.
Clearly, a need exists for solutions to the foregoing problems.